METHOD FOR PRODUCING STEEL STRIP AND COATED STEEL SHEET OBTAINED THEREFROM

Abstract

A method for producing a steel strip, the method comprising the following steps: hot rolling the steel strip into a hot-rolled strip; cold rolling the hot-rolled strip; hot-dip coating the cold-rolled strip with a Zn-based coating layer by guiding the cold-rolled strip into a bath containing molten zinc; and wiping the hot-dip coated strip after hot-dip coating using a gas knife having a knife slot from which a wiping gas is ejected, wherein the steel strip is cold rolled in a multi-stand cold rolling mill to a final cold-rolling thickness of 0.40 mm to 1.00 mm, and a cold-rolled and plated steel sheet comprises a steel substrate provided with a hot-dip metal coating layer.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a steel strip, the method comprising the following steps: hot rolling the steel strip into a hot-rolled strip; cold rolling the hot-rolled strip; hot-dip coating the cold-rolled strip with a Zn-based coating layer by guiding the cold-rolled strip into a bath containing molten zinc; and, after hot-dip coating, wiping the hot-dip coated strip using a gas knife having a knife slot from which a wiping gas is ejected; and a coated steel sheet comprising a steel substrate provided with a hot-dip metal coating layer obtainable by the method. [Background technology]

[0002] This type of process and the resulting products are widely known throughout the steel industry. Steel strip suitable for hot dip galvanizing is produced by hot rolling a steel slab into hot rolled strip, which is subsequently pickled and cold rolled in a multistand cold rolling mill into cold rolled strip, which is then galvanized in a continuous hot dip galvanizing line.

[0003] Continuous hot dip galvanizing lines are widely used and are found all over the world. Hot dip galvanizing was originally developed for galvanizing, i.e., coating with zinc, but is now also used to apply other metals or metal alloys to steel sheets.

[0004] In continuous hot-dip galvanizing, cold-rolled steel strip is passed as a continuous ribbon at high speed through a molten metal bath. In the bath, the steel strip reacts with the molten metal, and a coating bonded to the strip surface. The strip then passes through one or more submerged rolls and exits the bath vertically. Above the exit point, a set of gas knives wipes away excess molten metal, allowing for control of the coating thickness, typically expressed as coating weight per unit area of ​​the strip surface. After cooling, the strip enters the exit end of a hot-dip galvanizing line, which often includes a temper mill (also known as a skin-pass mill). Air or nitrogen gas is typically used as the wiping gas. Nitrogen gas is typically used to produce high-quality coated products.

[0005] Originally, hot-dip galvanized steel sheets were used in applications that did not require a high quality finish or a high degree of formability, but more recently they have been increasingly used in more demanding applications, such as automotive hoods, fenders, and doors.

[0006] The surface quality of galvanized steel sheets is affected by several types of defects. The main types of defects are dross-type defects, furnace defects, and coating defects, the latter of which are associated with the solidification and oxidation of the liquid metal during the hot dip coating process.

[0007] To improve surface quality, it is important to find ways to reduce not only dross-type defects and furnace defects, but also to reduce such plating defects. If such improvements are found, they will result in further improvements in the product, since other types of defects will become more noticeable and can be eliminated in a targeted manner. Furthermore, since other defects will no longer be overlooked, problematic plates can also be eliminated, resulting in a product with better overall surface quality being sold on the market.

[0008] Several methods have been proposed to improve the surface quality of the target product, particularly to reduce the above-mentioned plating defects. One proposed solution is to reduce the oxygen level in the atmosphere surrounding the steel strip after melting. Another proposed solution is to change the amount of specific elements, such as Al and / or Mg, present in the molten bath, or to add very specific elements, such as Be or Ga, to the bath.

[0009] Both solutions for improving the surface quality of galvanized sheet have drawbacks. The first solution requires the use of a containment box to shield the galvanized strip. Such a box limits the visibility of the strip and limits the space available for placing wiping devices and any additional devices (including skimming equipment), all of which are necessary for optimal control of the hot-dip galvanizing process. The second solution is often unsatisfactory because the application properties during use, such as susceptibility or corrosion resistance to filiform corrosion, are impaired. Summary of the Invention [Problem to be solved by the invention]

[0010] It is an object of the present invention to provide an improved method for producing hot-dip galvanized steel sheet having a high surface quality with a low number of defects and with low waviness in the parts that are visible in the final product, e.g., the body of an automobile.

[0011] It is also an object to provide an improved hot-dip galvanized steel sheet, which is suitable, inter alia, for use in the visible parts of motor vehicle bodies. [Means for solving the problem]

[0012] These objects are achieved by the independent claims. Preferred embodiments are defined in the respective dependent claims. It should be noted that the features recited in the claims can be combined in any technically meaningful way to describe further embodiments of the present invention. The following specification describes features of the present invention and contains additional embodiments of the present invention. Furthermore, it should be noted that features described in connection with the proposed method for producing steel strip can be used to further describe features of the proposed coated steel sheet, and vice versa. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 shows an XY plot of Sc and (1-5) in which the regions defined by the contours ABCDEA, A'FCDEA' and A"GCDEA" are shown. [Figure 2] Figure 2 shows the defective coil map for a coil with a length of 4084 m, a width of 1460 mm and a thickness of 0.6 mm. [Figure 3] FIG. 3 shows a defective coil map of a coil produced immediately after that shown in FIG. 2 with the same process settings (line speed, GKD) on a galvanizing line. DETAILED DESCRIPTION OF THE INVENTION

[0014] According to the present invention, in the method: The steel strip is cold rolled in a multi-stand cold rolling mill to a final cold roll thickness of 0.40 mm to 1.00 mm, and the cold rolling in the last stand is

number

[0015] It is clear that the upper and lower work rolls represent the two rolls in the rolling mill stand that come into contact with the strip being rolled.

[0016] Surprisingly, it has been found that in the production of this type of hot-dip galvanized steel strip, not only can the conditions of the hot-dip process step play a role in achieving the best surface quality of the product, but also the values ​​of the above parameters in the cold-rolling process step play a prominent role, which parameters in fact, according to the present invention, set entirely new standards for producing excellent surface quality on hot-dip galvanized steel products.

[0017] As a result, operation in accordance with the present invention not only reduces what are considered "coating" defects such as the above-mentioned localized slight wrinkles, but also results in a reduced presence of dross defects and numerous other defects under equivalent hot dip coating conditions when compared to various cold rolling regimens. It has been found that defects observed by camera inspection systems are significantly reduced when the present invention is applied, which can lead to improved production and higher yields in the production of the highest quality hot dip coated steel sheet.

[0018] In a further embodiment of the method according to the invention, the cold rolling in the last stand comprises:

number

[0019] It is advantageous if the cold rolling in the last stand is carried out using work rolls having a roughness Ra of 7 μm or less, preferably 6 μm or less, more preferably 5 μm or less, and in all cases a roughness Ra of 1.0 μm or more. As a result, good results are achieved within these roughness ranges, and the results are even better when the preferred ranges are used.

[0020] Where strip tracking is important, particularly to keep the strip well centered in the hot dip line, this roughness is preferably 3.0 μm or greater.

[0021] The surface roughness of the work rolls in the last stand can be produced by grinding and subsequent roll texturing by electrical discharge machining (EDT), which allows precise control of roughness parameters such as Ra and Rpc of the work rolls.

[0022] In one embodiment, the method is characterized by observing a GKD≦10 mm, where GKD is the average distance between the knife slot through which the wiping gas is ejected and the surface of the hot dip galvanized strip being wiped. While it is known that GKD plays a role in hot dip galvanizing related to producing a specific coating weight at a specific coating line speed using a specific pressure, it has been found that wiping knives with specific dimensions can produce products with good surface quality having GKD values ​​of 10 mm or less.

[0023] In preferred embodiments, GKD≦9 mm, GKD≦8 mm and GKD≦7 mm. Lower values ​​are preferred where possible, as they have been found to result in higher quality products, in particular as this allows for less waviness to be achieved in combination with fewer plating defects.

[0024] The strip may be stabilized by a magnetic device mounted near the ideal strip path between the bath and the first guide roll that contacts the strip downstream of the bath. The installation of such a device, for example in the form of an electromagnetic strip stabilizer, not only provides superior control of the hot dip coating thickness, but also allows for operation at a preferred lower GKD value without the risk of the strip contacting the wiping device, and for a more uniform coating weight distribution across the width of the strip.

[0025] In one embodiment, the composition of the bath of molten metal includes Zn, Al, and Mg, and the strip after coating and wiping is cooled in a cooling zone between the point where the strip is wiped and a downstream point where the strip first contacts the guide roll, and an active cooling gas flow rate Q(m) is required to maintain the strip temperature within a 20°C bandwidth of a target strip temperature of 200°C to 300°C at the guide roll. 3 / hr) is used, the cooling gas flow rate in the latter half of the cooling zone is a percentage p of Q, and the cooling gas flow rate in the first half of the cooling zone is a percentage (100-p) of Q, where p is set to be 70% or more.

[0026] It has been found that the higher p, the less waviness can be achieved in the product after plating. Premature cooling after wiping should be prevented as much as possible, and cooling should occur as late as possible while still reaching the desired maximum temperature of the strip before it contacts the guide roll (often called the upper roll). Therefore, p=80% or more, or even 90% or more, is preferred.

[0027] In one embodiment, the bath consists of 0.6-4.0 wt. % aluminum, 0.3-4.0 wt. % magnesium, a maximum of 0.2 wt. % each of elements belonging to the group of elements represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, the balance being unavoidable impurities and zinc.

[0028] The present invention has been found to work particularly well with such platings. The amount of elements belonging to the group of elements represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi may be up to 0.1% by weight for each element, or up to 0.05% by weight for each element.

[0029] In a further embodiment, the aluminum content is 0.6-3.0 wt%, preferably 1.0-3.0 wt%, more preferably 1.5-2.0 wt%, and / or the magnesium content is 0.3-2.0 wt%, preferably 1.0-2.0 wt%, more preferably 1.0-1.5 wt%. A relatively high Mg content provides better corrosion protection. A lower Al and Mg content provides better weldability and a reduction in the surface feature known as the "marble effect", which can appear due to the solidification and oxidation behavior of Zn-Al-Mg coatings.

[0030] In an alternative embodiment, the bath comprises 0.25-0.90 wt. % aluminum, preferably 0.25-0.50 wt. % aluminum, and up to 0.2 wt. % each of elements from the group represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr, and Bi, the balance being unavoidable impurities and zinc. The amount of elements from the group represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr, and Bi may be up to 0.1 wt. % for each element, or up to 0.05 wt. % for each element.

[0031] Such plating in itself already results in an improvement in the surface quality of the plated steel sheet, so it is advantageous to produce plated steel sheet by the method of the present invention and obtain a product with excellent properties.

[0032] In embodiments where the hot dip coated strip is temper rolled with an elongation of 0.5% or more, temper work rolls are used having an average diameter of 400 mm or more, more preferably 500 mm or more, and even more preferably 600 mm or more, where average diameter is defined herein as the average diameter of the upper and lower work rolls at the center position of the rolls.

[0033] Such a combination of elongation and tempering work roll diameter is beneficial for surface quality and roughness transfer.

[0034] In a preferred embodiment, the temper mill uses temper work rolls having a roughness Ra of 4.5 μm or less, preferably 3.0 μm or less, more preferably 2.5 μm or less, which realizes less waviness and a higher number of peaks in the temper-rolled coated steel sheet, which is beneficial to the appearance of painted parts made from the coated steel sheet.

[0035] The present invention also provides a plated steel sheet obtainable by the method, comprising: The plated steel sheet comprises a steel substrate provided with a hot-dip metal plating layer, The thickness of the steel substrate is 0.40 mm to 1.00 mm; i) the steel substrate has the following composition, all by weight: C: maximum 0.04; Mn: 0.01~1.20; Si: 0.001~0.50; Al: 0.005~0.1; P: max 0.15; S: max 0.045; N: max 0.01; Mo: max 0.12; Ti: max 0.12; Nb: max 0.12; Cu: max 0.10; Cr: max 0.06; Ni: maximum 0.08; B: maximum 0.0025; V: max 0.01; Ca: max 0.01; Co: max 0.01; Sn: max 0.01; Iron and unavoidable impurities: balance and ii) The plated steel sheet has a surface characteristic Sc, and Sc is Sc=Sk / (0.7*t+0.3) [In the formula, Sk (μm) is defined in accordance with NEN-EN-ISO 25178-2:2012, t is the thickness of the steel substrate (mm). is defined as follows: iii) the waviness Wsa of the coated steel sheet measured in the rolling direction according to SEP 1941 after 5% Marciniak biaxial deformation is Wsa(1-5) value (μm); iv) the combination of Sc and Wsa lies within the region defined by the contour ABCDEA in the XY plot of Sc and Wsa, respectively; where: A is Sc=3.00 and Wsa=(0.2686)-(0.0543*Sc)+(0.0105*Sc ∧ 2) is defined as the intersection with AB is from Sc=3.00 at A to Wsa=0.50 at B, Wsa=(0.2686)-(0.0543*Sc)+(0.0105*Sc ∧ 2) is defined by BC is defined from B to C where Sc=14.50 and Wsa=0.50; CD is defined by Wsa=0.50 at C to Wsa=0.10 at D, Sc=14.50, DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; EA is embodied in a galvanized steel sheet that closes the contour and is defined by E to A, Sc=3.00.

[0036] As a result, when a steel sheet comprising a steel substrate provided with a hot-dip metal coating layer according to the present invention has the above-mentioned characteristics, the hot-dip coated steel product has a very good surface quality in its final application, for example as the visible side of a body part of an automobile. Sk as used in this patent document refers to a surface property parameter, also named "core roughness", measured in accordance with NEN-EN-ISO 25178-2:2012.

[0037] In the experiment, Sk was measured using a confocal microscope with WinSam 2.6 software, which filters the measurement data and calculates Sk. The details for Sk measurement were as follows: equipment from Nanofocus; instrument type μSurf Mobile (also known as Marsurf mobile); objective lens MPlanApo N 800XS (20x / 0.60); lateral spacing [μm] 1.56; number of stitch areas 3*3; measurement area 2.1*2.1 mm; software WinSam 2.6; calculation / evaluation area 2.0*2.0 [mm]; filter quadratic polynomial; penetration depth (kfl max) +10 [μm]; penetration depth (kfl min) -10 [μm]; number of steps 2000; step width 10 [nm].

[0038] Sk may be measured with equipment similar to that available commercially and with similar software.

[0039] In a preferred embodiment, the combination of Sc and Wsa lies within a region defined by a contour A'FCDEA' in an XY plot of Sc and Wsa, respectively; where: A' is Sc=3.00 and Wsa=(0.2276)-(0.0266*Sc)+(0.0054*Sc ∧ 2) is defined as the intersection with A'F ranges from Sc=3.00 in A' to Wsa=0.50 in F, Wsa=(0.2276)-(0.0266*Sc)+(0.0054*Sc ∧ 2) is defined by FC is defined from F to C where Sc=14.50 and Wsa=0.50; CD is defined by Wsa=0.50 at C to Wsa=0.10 at D, Sc=14.50, DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; EA' closes the contour and is defined from E to A' by Sc=3.00.

[0040] This results, among other things, in hot dip galvanized steel products with better surface quality in the end use.

[0041] In a more preferred embodiment, the combination of Sc and Wsa lies within an area defined by contour A "GCDEA" in an XY plot of Sc and Wsa, respectively; where: A” is Sc=3.00 and Wsa=(0.208)-(0.0118*Sc)+(0.0027*Sc ∧ 2) is defined as the intersection with A”G” ranges from Sc=3.00 in A” to Wsa=0.50 in G, Wsa=(0.208)-(0.0118*Sc)+(0.0027*Sc ∧ 2) is defined by GC is defined from G to C where Sc=14.50 and Wsa=0.50; CD is defined by Wsa=0.50 at C to Wsa=0.10 at D, Sc=14.50, DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; EA" closes the contour and is defined from E to A" by Sc=3.00.

[0042] This results in a hot dip galvanized steel product with optimum surface quality in the final application.

[0043] In a preferred embodiment, the board has a combined weight of 60 to 175 g / m 2 The coating weight is measured in accordance with EN 10346:2015. The lower the coating weight, the less waviness can be achieved by hot dip coating.

[0044] In one embodiment, the surface roughness Ra of the plated steel sheet is 0.9 μm to 1.8 μm, preferably 0.9 μm to 1.6 μm, and more preferably 0.9 μm to 1.4 μm, measured in accordance with ISO-NEN 468-1982 with a 2.5 mm cutoff. These roughness values ​​allow for good waviness after deformation.

[0045] The present invention is also embodied in a method as described above, which is carried out for the purpose of producing hot-dip galvanized steel sheet having a guaranteed maximum waviness Wsa in its final deformation state in its final use, characterized in that the guaranteed maximum waviness Wsa, measured in the rolling direction, has a Wsa(1-5) value of 0.35 μm, 0.34 μm, 0.33 μm, 0.32 μm, 0.31 μm, 0.30 μm, 0.29 μm, 0.28 μm or less in accordance with SEP 1941. It was particularly surprising to find that a measure measured upstream in the production process, such as by cold rolling, allows for the realization of such an objective, which is of great importance in connection with the final use, for example in the visible body of an automobile.

[0046] The invention will now be explained in more detail using figures and experimental descriptions.

[0047] In the diagram: FIG. 1 is an XY plot of Sc and (1-5) showing the region defined by the contours ABCDEA, A'FCDEA' and A"GCDEA", and the experimental combinations of Sc and Wsa that are inside and outside the present invention. Figure 2 shows the defect coil map for a coil with a length of 4084 m, a width of 1460 mm, and a thickness of 0.6 mm. For this coil, 93.4% was classified as acceptable for surface-critical applications. The remaining coil had surface defects with too high a local density, which corresponds to a surface quality grade of "++" according to Table 2. This coil was processed in a cold rolling mill with SRF=6130 kN / m and AWR=474 mm. Figure 3 shows the defect coil map of a coil produced immediately after the one shown in Figure 2, with the same process settings (line speed, GKD) on a galvanizing line. This coil was 4004 m long, 1460 mm wide and 0.6 mm thick. For this coil, 75.5% was classified as acceptable for surface-critical applications, which corresponds to a surface quality grade of "+" according to Table 2. This coil was processed with an SRF of 5052 kN / m and an AWR of 430 mm. [Example]

[0048] To carry out the experiments, the samples were prepared by casting steel slabs, then hot rolling the slabs in a hot rolling mill to obtain hot-rolled steel strips, processing the hot-rolled steel strips in an acid cleaning line, cold rolling the acid-cleaned steel strips in a cold rolling mill, annealing, hot-dipping the cold-rolled strips, and temper rolling in a temper mill (also called skin-pass in a skin-pass rolling mill).

[0049] Unless otherwise specified in the tables or text, the manufacturing process settings up to and including hot rolling were in accordance with normal practice.

[0050] Over the course of the experiments, steel substrates made from different steel casts were used, with the compositions shown in Table 1 below.

[0051] [Table 1]

[0052] The hot rolled samples, strip material, were cold rolled in the same cold rolling campaign at the same time and hot-dipped according to the same hot-dipping regime. The main data for the hot-dipping process were as follows:

[0053] For the production of galvanized material ("GI"), Zn baths with a target aluminum content of 0.30% to 0.40% were used.

[0054] For the production of so-called zinc-magnesium plated materials ("ZM"), Zn baths were used with a target Mg content of 1.45%-1.50% and an aluminum content of 1.70%-1.75%. In practice, the amount of Mg in the bath varied from 1.40% to 1.70%, and the amount of Al varied from 1.60% to 1.80%.

[0055] Unless otherwise indicated, the knife slot width was 1.2 mm. The gas knife distance GKD was varied from 7 mm to 10 mm.

[0056] The production of the hot dip galvanized steel sheet examples was done in batches. Within a batch, coils of similar steel composition, thickness, and width were produced one after the other. The quality of the strip was determined by visual inspection supported by camera inspection of the strip to assess the amount and severity of any imperfections on the strip surface. The grades used in the following example descriptions are shown in Table 2.

[0057] [Table 2]

[0058] Two example camera inspection defect maps (see Figures 2 and 3) show the sudden changes in surface quality that can occur in the final stands of a cold rolling mill as coil after coil is produced using various rolling forces. Each dot represents a surface feature classified as a defect by the camera inspection system across the width and length of the strip surface. The maps show both the bottom (left) and top (right) surfaces of the strip. In this case, the top surface is the visible surface in surface-critical applications. The majority of the defects shown are classified as dross-type defects.

[0059] During the search for the best conditions to achieve excellent surface quality in terms of waviness and low defect counts, it was observed that the number of defects detected by the camera inspection system could vary greatly between strip coils, and that some combinations of steel composition, thickness, and pre-processing tended to perform worse than others. Strip coils with a large number of defects were rejected during the inspection process. Examples are shown in Table 3 below, where it can be seen that graded Examples 2.1 and 2.3 were completely rejected.

[0060] [Table 3]

[0061] Operators usually expect that the cause of these quality deviations is the hot dip coating process. They change the process settings of the hot dip coating line to improve the quality so that the quality complies with the specifications. In such cases, changes in line speed, fluctuations in bath level, and variations in furnace temperature or temper mill processing are suspected as possible causes of the deviations.

[0062] Based on the experimental results, it was observed that product quality varied despite constant hot dip coating conditions, and this variation appeared to be correlated with the cold rolling regimen. Coils processed according to one cold rolling regimen demonstrated a higher number of surface defects than coils processed according to another cold rolling regimen.

[0063] To analyze the role of the cold rolling regimen on the surface quality of hot dip coated products, the substrate surface properties on the steel substrate surface beneath the coating were measured.

[0064] To accomplish this, the coating and the blocking layer of hot-dip galvanized samples having a size of 20 mm x 20 mm were peeled off from the steel substrate. 800 mL of water; 155 mL of a water-based solution of hydrochloric acid containing 37.5% by volume of hydrochloric acid; and 1 mL of Leuzolit® Extra 283-M, a commercially available over-acid wash inhibitor The acid wash was carried out by placing the samples, in batches of up to six samples, lengthwise into an acid wash solution prepared by mixing:

[0065] Leuzolit® inhibitor was added to ensure that the steel substrate was not etched or pickled by the hydrochloric acid and that the pickling did not substantially affect the surface texture or roughness of the steel substrate. During this pickling process, gases were evolved that escaped from the pickling bath to the surface of the pickling solution. The pickling was continued until gas evolution had nearly ceased, which typically took 10-15 minutes.

[0066] Since the surface texture of the steel substrate can be affected by temper rolling of the plated steel, the inventors focused on the so-called core roughness of the substrate as this is more representative of the original cold rolled strip surface.

[0067] The core roughness Sk was measured according to the standard method mentioned above.

[0068] For the examples in Table 4, it was found that despite the hot rolling process, and overall cold rolling reduction, substrate thickness and width being substantially the same, there were significant differences in surface texture between the various quality levels. Substrates that exhibited higher surface core roughness had the best surface quality, as tracked by the lower number of defects recorded by camera inspection and visual inspection.

[0069] Details of an example illustrating this are shown in Table 4 below.

[0070] [Table 4]

[0071] During further production runs, it was observed that although the associated hot dip coating conditions remained constant, the quality of the coated product still correlated with the cold rolling regimen, as can be seen in Table 5.

[0072] [Table 5]

[0073] Upon further consideration of processing conditions for the cold rolling mill, the inventors noticed differences in deformation in the final stand associated with the use of various specific rolling forces, which are defined as the total rolling force applied divided by the width of the strip in combination with various work roll radii. Higher rolling forces combined with smaller work roll diameters resulted in better surface quality, while lower rolling forces combined with larger work roll diameters resulted in poorer surface quality.

[0074] Further monitoring of the cold rolling regimen for the last stand was carried out, particularly with regard to the specific rolling force and work roll radius, and further tests were carried out to evaluate the exact influence of cold rolling in the last stand of the cold rolling mill. The results of these tests are shown in Table 6. Clearly, a reduction in rolling force has a negative effect on the surface quality of the hot-dip galvanized steel. The number of defects in the coil is significantly higher with a lower specific rolling force in the last stand, and the best quality is achieved for coils produced with a higher specific rolling force in the last stand of the cold rolling mill.

[0075] [Table 6]

[0076] As a next step, tests were conducted in which the specific rolling force applied to the last stand was increased beyond the normal range that a cold rolling mill operator would normally use. The data from these tests are given in Table 7.

[0077] [Table 7]

[0078] These results show that increasing the rolling force significantly increases the surface quality by significantly reducing the number of defects on the surface of the strip.

[0079] The results of the tests using work roll diameters and rolling forces are shown in Table 8.

[0080] [Table 8]

[0081] The results of this test clearly show that a combination of high specific rolling force and smaller average work roll radius results in a lower number of defects, and also that a higher specific rolling force divided by roll radius provides a better opportunity for good surface quality.

[0082] Because the roughness transfer during cold rolling is higher for thicker and / or softer materials and lower for thinner and / or harder materials, the same final stand processing in terms of specific rolling force and average work roll radius can result in different core roughness for materials of various gauges or strengths. Therefore, for thinner and / or stronger materials, a further increase in rolling force is required to achieve the same core roughness as for thicker and / or softer materials. Nevertheless, increasing the specific rolling force and / or reducing the average work roll radius in the final stand improves the opportunity for superior surface quality.

[0083] The difference between the roughness transfer for different materials is illustrated in Table 9, where thinner rolled materials with higher specific rolling force and similar diameter have lower core roughness. Taking this into consideration, the surface property Sc=Sk / (0.7*t+0.3), where t is the thickness of the steel substrate in mm, is introduced as a measure to better compare the effectiveness of the final stand rolling process for different material thicknesses.

[0084] [Table 9]

[0085] In a further attempt to maintain a low level of surface defects detected by the camera inspection system and to improve the waviness of the coated steel sheet after deformation, the inventors also tested the effect of rolling force with lower work roll roughness, the results of which are shown in Tables 10 and 13.

[0086] The waviness after deformation was established by measuring Wsa(1-5) (μm) in the rolling direction according to SEP 1941 after biaxially deforming the specimen by 5% using the Marciniak instrument.

[0087] This proved equally useful, resulting in excellent waviness after deformation and low defect detection by the camera system, as will be seen in the examples below.

[0088] [Table 10]

[0089] The waviness produced is not only independent of the work roll roughness in cold rolling mills, but also independent of the gas knife distance (GKD) and the cooling conditions after wiping. Tests by the inventors have shown that the shorter the knife distance and the later the cooling of the strip after it leaves the zinc pot, the less waviness of the coating produced after deformation. This is illustrated by the examples in Tables 11 and 12.

[0090] [Table 11]

[0091] [Table 12]

[0092] The experiments shown in Table 12 were carried out with the active cooling gas flow rate (m from the blower) used in the second half of the cooling tower. 3The active cooling gas flow rate, also referred to as Q in this document, represents the active cooling gas flow rate required to maintain the strip temperature within a 20°C bandwidth of the target strip temperature of 230°C at the first roll in the cooling tower through which the strip passes after the gas knife. In this example, the total flow rate, Q, is the sum of the flows from coolers 1 through 4. Coolers 3 and 4 in this example are located in the latter half of the cooling tower; therefore, the total flow rate of coolers 3 and 4 divided by Q and multiplied by 100 equals p. The results show that applying a larger percentage, p, of Q to the latter half of the cooling tower improves strip waviness by allowing the strip to cool as slowly as possible in the first half.

[0093] These results show that the present method can be used to produce excellent hot dip galvanized strip with a low number of defects and very low levels of waviness after deformation.

[0094] Based on the overall study and the examples in Table 13 below, it was concluded that the combination of Sc and Wsa within the contour ABCDEA shown in Figure 1 results in a high quality hot dip galvanized product and allows for a very effective manufacturing process.

[0095] [Table 13]

[0096] If the combination of Sc and Wsa falls within the contour A'FCDEA', the product is better, and if it falls within the contour A"GCDEA", it is still better.

[0097] Lines EA, EA' and EA" represent the roughness required to allow proper strip tracking in the hot dip galvanizing line, preventing slippage and scratches.

[0098] Lines BC, FC and GC show the maximum waviness, above which the appearance of the steel paint, when finally painted, is no longer sufficient for use in high quality visible components.

[0099] The line CD represents the maximum value Sc, above which the advantages of the invention are negated by the fact that firstly the average roughness will be higher than achievable for the plating weight desired by the customer, and secondly for thin platings very high wiping pressures will be required to control the plating weight.

[0100] In the course of this experiment, the use of the cold rolling regimen according to the present invention was also tested on GI, and it was confirmed that the number of defects was reduced by the present invention for other types of plating besides ZM as well. The results are shown in Table 14.

[0101] [Table 14]

[0102] The present invention provides a ceramic ceramic having the following content, all in weight percent: C up to 0.04, or up to 0.01, or up to 0.007, and / or Mn up to 1.2, or up to 0.80, and / or Si up to 0.50, or up to 0.30, and / or Al up to 0.1, or up to 0.08, and / or P up to 0.15, or up to 0.10, and / or S up to 0.045, or up to 0.020, and / or N up to 0.01, or up to 0.008, or up to 0.004, and / or Ti up to 0.12, or up to 0.0 and / or Nb up to 0.12 or up to 0.03, and / or Mo up to 0.12 or up to 0.01, and one or more of the optional elements: Cu up to 0.10 or up to 0.08, Cr up to 0.06 or up to 0.04, Ni up to 0.08, B up to 0.0025 or up to 0.0015, V up to 0.01 or up to 0.004, Ca up to 0.01, Co up to 0.01, Sn up to 0.01, the balance being iron and unavoidable impurities.

[0103] In this patent document it is stated that the Ra of a surface denotes its roughness according to ISO-NEN 468-1982 at a cut-off of 2.5 mm.

[0104] It is further stated that the waviness value Wsa is Wsa(1-5) (μm) determined in accordance with SEP 1941:2012-05 in the rolling direction (also referred to herein as "rd") of the strip and, where appropriate, after 5% Marciniak biaxial deformation.

[0105] Finally, the * in the formula represents multiplication, ∧ It is stated that " represents a power.

Claims

1. 1. A method for producing high surface quality steel strip for use in an automobile body, comprising the steps of: The method comprises the steps of: hot rolling the steel strip into a hot rolled strip; cold rolling the hot rolled strip; hot-dip coating the cold-rolled strip with a Zn-based coating layer by directing the cold-rolled strip into a bath containing molten zinc; After hot-dip coating, wiping the hot-dip coated strip using a gas knife having a knife slot through which a wiping gas is ejected. Including, The steel strip is cold rolled in a multi-stand cold rolling mill to a final cold roll thickness of 0.40 mm to 1.00 mm; Cold rolling in the last stand [Equation 1] [In the formula, SRF is the specific rolling force (kN / m) calculated by dividing the rolling force (kN) by the strip width (m); AWR is the average work roll radius (m) of the upper and lower work rolls at the center position of the rolls. It is carried out so that GKD, which is the average distance between the knife slot through which the wiping gas is ejected and the surface of the hot-dip galvanized strip to be wiped, is GKD≦10 mm; the composition of the bath of molten metal includes Zn, Al and Mg; the strip after plating and wiping is cooled in a cooling zone between the position where the strip is wiped and a downstream position where the strip first contacts the guide roll; The active cooling gas flow rate Q (m 3 / hr) required to maintain the strip temperature at the guide roll within a 20°C bandwidth of the target strip temperature of 200°C to 300°C is used; the cooling gas flow rate in the latter half of the cooling section is a percentage p of Q; the cooling gas flow rate in the first half of the cooling zone is a percentage of Q (100-p); A method wherein p is set to 70% or more, 80% or more, or 90% or more.

2. The value of SRF / AWR is [Equation 2] The method according to claim 1, wherein the order of preference is:

3. 3. The method according to claim 1 or 2, wherein the cold rolling in the last stand is carried out using work rolls having a roughness Ra of 7 μm or less, 6 μm or less, or 5 μm or less, and in all cases a roughness Ra of 1.0 μm or more.

4. The method described in claim 3, wherein in all cases the roughness Ra is 3.0 μm or more.

5. 5. The method according to claim 1, wherein GKD, which is the average distance between the knife slot through which the wiping gas is ejected and the surface of the hot-dip galvanized strip to be wiped, is GKD≦9 mm, GKD≦8 mm, or GKD≦7 mm.

6. Bath, 0.6 to 4.0 wt. % aluminum, 0.3 to 4.0 wt. % magnesium; optionally up to 0.2% by weight of each of elements belonging to the group of elements represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, The balance is unavoidable impurities and zinc The method according to any one of claims 1 to 5, comprising:

7. 7. The method of claim 6, wherein the aluminum content is 0.6 to 3.0 wt. %, 1.0 to 3.0 wt. %, or 1.5 to 2.0 wt. %, and / or the magnesium content is 0.3 to 2.0 wt. %, 1.0 to 2.0 wt. %, or 1.0 to 1.5 wt. %.

8. Bath, 0.20 to 0.90 wt. % aluminum, or 0.25 to 0.50 wt. % aluminum; elements belonging to the group of elements represented by Pb, Sb, Ti, Ca, Mn, Sn, La, Ce, Cr, Ni, Zr and Bi, each of which is at most 0.2% by weight, The balance is unavoidable impurities and zinc The method according to any one of claims 1 to 4, comprising:

9. 9. The method according to claim 1, wherein the hot-dip galvanized strip is temper rolled to an elongation of 0.5% or more using temper work rolls having an average diameter of 400 mm or more, 500 mm or more, or 600 mm or more.

10. The method according to claim 9, wherein a tempered work roll having a roughness Ra of 4.5 μm or less, 3.0 μm or less, or 2.5 μm or less is used.

11. A plated steel sheet obtained by the method according to any one of claims 1 to 10, The plated steel sheet comprises a steel substrate provided with a Zn-based hot-dip plating layer; The thickness of the steel substrate is 0.40 mm to 1.00 mm; i) a steel substrate having the following composition, all in weight percent: C: maximum 0.04; Mn: 0.01-1.20; Si: 0.001 to 0.50; Al: 0.005-0.1; P: maximum 0.15; S: maximum 0.045; N: maximum 0.01; Mo: max 0.12; Ti: maximum 0.12; Nb: maximum 0.12; Cu: maximum 0.10; Cr: maximum 0.06; Ni: maximum 0.08; B: maximum 0.0025; V: maximum 0.01; Ca: maximum 0.01; Co: maximum 0.01; Sn: maximum 0.01; Iron and unavoidable impurities: balance and ii) The plated steel sheet has a surface property Sc, and Sc is Sc=Sk / (0.7*t+0.3) [In the formula, Sk (μm) is defined in accordance with NEN-EN-ISO 25178-2:2012, t is the thickness of the steel substrate (mm). is defined as follows: iii) After 5% Marciniak biaxial deformation, the waviness Wsa of the plated steel sheet measured in the rolling direction in accordance with SEP 1941 is Wsa(1-5) value (μm), iv) the combination of Sc and Wsa lies within the region defined by the contour ABCDEA in an XY plot of Sc and Wsa, respectively; where: A has Sc = 3.00 and Wsa = (0.2686) - (0.0543 * Sc) + (0.0105 * Sc ∧ 2) and is defined as the intersection point with AB is from Sc=3.00 in A to Wsa=0.50 in B, Wsa=(0.2686)-(0.0543*Sc)+(0.0105*Sc ∧ 2) is defined by BC is defined from B to C where Sc=14.50 and Wsa=0.50; CD is defined by Wsa = 0.50 at C to Wsa = 0.10 at D, Sc = 14.50; DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; EA is a galvanized steel sheet that closes the contour and is defined by E to A, Sc=3.

00.

12. The combination of Sc and Wsa lies within the region defined by the contour A'FCDEA' in the XY plot of Sc and Wsa, respectively; where: A' is Sc = 3.00 and Wsa = (0.2276) - (0.0266 * Sc) + (0.0054 * Sc ∧ 2) and is defined as the intersection point with A'F is from Sc=3.00 in A' to Wsa=0.50 in F, Wsa=(0.2276)-(0.0266*Sc)+(0.0054*Sc ∧ 2) is defined by FC is defined from F to C where Sc = 14.50 and Wsa = 0.50; CD is defined by Wsa = 0.50 at C to Wsa = 0.10 at D, Sc = 14.50; DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; The plated steel sheet according to claim 11, wherein EA' closes the contour and is defined by E to A', Sc = 3.

00.

13. The combination of Sc and Wsa lies within the region defined by the contour A "GCDEA" in the XY plot of Sc and Wsa, respectively; where: A" is Sc = 3.00 and Wsa = (0.208) - (0.0118 * Sc) + (0.0027 * Sc ∧ 2) and is defined as the intersection point with A"G from Wsa = 0.20 to Wsa = 0.50 in G, Wsa = (0.208) - (0.0118 * Sc) + (0.0027 * Sc ∧ 2) is defined by GC is defined from B to C where Sc=14.50 and Wsa=0.50; CD is defined by Wsa = 0.50 at C to Wsa = 0.10 at D, Sc = 14.50; DE is defined by Sc=14.50 at D to Sc=3.00 at E, Wsa=0.10; 12. The plated steel sheet of claim 11, wherein EA" closes the contour and is defined by E to A" Sc = 3.

00.

14. Both sides combined: 60 to 175 g / m ２ The plated steel sheet according to any one of claims 11 to 13, having a total plating weight of

15. The plated steel sheet according to any one of claims 11 to 14, wherein the plated steel sheet has a surface roughness Ra of 0.9 µm to 1.8 µm, 0.9 µm to 1.6 µm, or 0.9 µm to 1.4 µm.

16. 11. The method according to any one of claims 1 to 10, which is carried out with the aim of producing a hot-dip galvanized steel sheet which, in the deformed state in its final use, has a guaranteed maximum waviness Wsa, characterized in that the guaranteed maximum waviness Wsa, measured in the rolling direction according to SEP 1941, is a Wsa(1-5) value of 0.35 μm, 0.34 μm, 0.33 μm, 0.32 μm, 0.31 μm, 0.30 μm, 0.29 μm, 0.28 μm or less.

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